Electrodeposition of chromium



May 9, 1950 R. R. LLOYD ELEcTRoDEPosITIoN oF CHROMIUM Filed Feb. 4, 1946 ZOFDJOm 5 w: Iz

Patented May 9, 11950 ELECTRODEPOSITION OF CHROMIUM Rex R. Lloyd, `Boulder City, Nev., assignor, by

mesne assignments, to Crimora Research & Development Corporation, New York, N. Y., a corporation of Virginia Application February 4, 1946, Serial No. 645,474

(Cl. 21M- 105) 2 Claims.

The present invention relates to the prepara-V tion of chromium sulfate solution and, more particularly, to the formation from chromite ore of a.- chromium sulfate solution in a sufficiently pure state so as to render the same suitable for the preparation of chromium plating electrolytes. This application is a continuation in part of application Ser. No. 578,550, filed February 17, 1945, now abandoned. e Y

Heretofore, it has been the practice to dissolve chromite in sulfuric acid and chromic acid, thereby oxidizing the iron of the chromite. Such a solution, however, is unfit for use in chromium plating electrolytes, for example, until the iron and other impurities are removed. Prior methods of removing iron and other impurities from the solution generally have been unsatisfactory.

It is an object of the present invention to'provide a satisfactory method of removing iron and other impurities such as aluminum and magnesium from the chromium sulfate solution.

Another aim of the invention is to obtain a solution containing chromium sulfate stabilized in the green modification form.

A further aim of the invention is to provide a method of preparing chromium sulfate solutions from chromite which is suitable for use with an electroplating step whereby the electroplating may be carried out continuously, if desired.

Another aim of the invention is to provide a method of preparing chromium sulfate solution from chromite for use with an electroplating operation whereby the products of electrolysis, i. e., spent electrolyte, may be re-used to form the chromium sulfate solution.

Other advantages and aims of the invention will be apparent from the following description and appended claims.

The figure is a flow sheet illustrating the practice of the method.

" In accordance with the present invention, the

.chromite ore preferably is first ground to a ne state, for example, by Wet grinding to less than 200 mesh size, filtering and drying, to facilitate treatment. The finely ground chromite, so prepared, is then digested with sulfuric and chromic acids. This step is indicated in box I of the flow sheet. For preferred results, the amount of sulfuric acid used is sufiicient to convert all of the metal in the ore to sulfates, while the amount ot chromic acid used is slightly in excess of that required to oxidize the iron. In general, a concentrated sulfuric acid of the order of about 65% H2804, is required for efficient digestion. Heating 2 to an elevated temperature to about C. also facilitates digestion.

After digestion has been carried out to a desired degree, preferably to a point where substantially all of the metal is dissolved, the solution of metal sulfates is separated from the undis-A solved residue in any convenient manner, such as by filtering. This step is indicated in box 2 of the flow sheet. Any undissolved chromite will be retained on the filter along with the residue. Such undissolved chromite may be separated from the residue by known ore dressing means and returned to the digestion step, as indicated by the lines on the flow sheet.

As will be explained more fully hereinafter, it, is an advantage of the present invention that the chromic acid and a large part of thesulfuric acid` used in the digesting step can be recovered and. re-used, when the resulting material is used as an electrolyte for electrowinning chromium. This is indicated by the line running from the cell IE! to the digester I. Some sodium and ammonium sulphates may be added to the digester solution.

The filtrate obtained after the leaching operation is then preferably combined with additionalA ammonium and sodium sulphates and heated to a temperature above '70 C. to stabilize the chromic salts in the green modification, a form wellknown in the art. This step is indicated by box 3 in the fiow sheet.

Following the stabilization of the chromic salts in the green modification, as described above, it is an advantage of the present invention that the undesired constituents of the solution may be readily separated from chromic sulfate in the stabilized green modification. In accordance with' the invention, this is accomplished by cooling the solution to a low temperature, such as '-5 C. to 10 C. Under such temperature conditions, the iron, aluminum and magnesium present will sepa--A rate as crystals of complex salts and may be removed, for example, by centrifuging or filtering. These steps are indicated by boxes 4 and 5, re spectively, in the fiow sheet.

If desired, the resultant partially purified solution containing chromium sulfate may be further improved for use in a chromium electrowinning process by converting the chromic sulfate present into its violet modification. The separation of the chromic sulfate may be effected by crystallization of the violet chromic sulfate and may d indicated by box 8 in the ow sheet. 4They are now in a form for use in a chromiunielectrowinning process, such as is described in my copending application, Ser. No. 545,473, filed February 4, 1946, wherein sulphates .off chrom13ml,l

ammonium and sodium are employed. Asstated in said application, these components are present in the electrolyte in the following proportions:

Sodium -40 grams perliter Ammonium 30..- 50 grainsper liter Chromium 13.6,0.gramsper.liter Thesulfate ion is present in an amount at least Stoichiometrically equivalent to the sodium, .ammonium and chromium. The .amounts Aof the components added tofform the Afeedrsolution -are governed accordingly-te replenish the electrolyte To increase their purity, they may be re-crys.- tallizedone or more times. This step'is-.indicated inbox Qof the :flow sheet.

TheY conversion to the violet modification may be catalyzed by the addition of chromous sulfate. Such chromous suifate may be obtained by utilizing nearly all of the spent catholyte from the electrowinning cell. amount to expedite the crystallization of the violetCr+++ salts and to improve vtheir purity, and the ,quantityis not critical. This step is indicated by the line running from the cel bei; I0 4to'box 9 marked spent catholyte. A part of the spent catholyte is not necessary igor this purpose and is mixed with the anolyte in the cell, where it serves to reduce part of the Ghromio acid formed.

The pure crystals are especially suitable for replacing the chromium in the electrolytavwhich,

has been plated o-ut in the electrowinning opera tion. This replenishment is Vaccoifnplished .by mixing the crystals with a small amount of makeup sodium sulphateand heating to above 70 C. to stabilize-the Cr+++ salts in the greenmodication. This step. iS indQatedbytheiline :lill-naine from box-9 .to thestabiliger, box il and thence to thecathode compartment of the ,cell IE.

The method of thepresent inventionmaybe usedftopermit continuous orcyclic operation by direct replacement of plated outcln'omium from readily available chromite ore. The productsof electrolysis, other than metal; namely,chro1nous sulfate, chrcmic and sulfuric acidsmay berecycled in accordance With the invention, asile.- scribed above and shown on the flow sheet, for economyand convenience of operation.

The practice of the invention is ycapable of many Variations by one skilled in the art Without departing from the teachings herein, and all such It is added in sufficientvariations are intended to be included Within the scope of the following claims.

What is claimed is: Y

1. In a process for the preparation of electr lytic chromium utilizing a compartment cell and an electrolyte preparedby treating chr'omite with SbllphilriY acid and chromic; acid,L the steps of uti-lizinghsulphuric acid and chromic: acid lderived from the anolyte of the compartment cell to prc vide at least part of the necessary sulphuric and chromic acids, digesting the acids and chromite at approximately C., separating the chromic sulfate andother metallic sulfates dissolved from the -chromi-te from the insoluble residue, adding ainifnoniurnv sulfate in an amount sufficient to form ammonium sulfate complexes of the chromiumand otherinetallic sulfatos present, heating thesolution to a temperature above '70 C. andV belowthe temperature at which decomposition occurs, whereby to-:stabilize the chromium salts the `green modification, .coolinga'fihe @Solutionto *a temperature kbetween 5 andY e.- to crystallize` iron, aluminum.and;,in-,agnesiw ammonium sulfate4 complexes and; s elazrli them from the solution` ofl green chromium `sul-1,.y fate, seeding the-solutiontOnbtain thQ- Qhlili salts in the violet modification, adding` a su stantial 'fraction of the spent oatholyte, t `Ae; solution to catalyze the crysifa-llizatiozlll 0i ,Qlei chromium alum. from the solution, reedissolyipg the-crystallized chromium-alum tolform annonce ous solution, mixing thev chrom ium .a1li 1m Milla.- sodium sulfate in an amount .slightly .le litten;y the ammonium content of the. alum to f0rm l catholyte solution having 20e40grams per ofilsodium, 30-501gramsper liter of.v ammo mi, and 18-60 -gramsperliter of chromium, al, the for-m of sultates, yheating the Y,lcatljlolyte solution toY above 70?-V C. andl below.- theiemperatuge; atowhich` deeompositionzoccurs to stabilize greenmodication, electrolyzing suchsollllril in the. catholyte. compartment Q -i aneletr ytic such cell chromic and .S1111- 2. 111th@ jpracess asset tor-th in Claim 1, the additional'. step ,o f.4 centrusius the. catala/Zed; solution tn Separant-e lthe violet chromiurn therefrom- REX R..

REFERENCES' Cir- ED- -frhe following. fxeferences archi record. the! iile .of this patent:

Y UNITED STATES` .PATENTES Menor, Compramos@ treatise en lnereseb. andv 'IlieQietical Chemistry, v ol. 1931), pp.' 43.5; L3-'7l .438i 

1. IN A PROCESS FOR THE PREPARATION OF ELECTROLYTIC CHROMIUM UTILIZING A COMPARTMENT CELL AND AN ELECTROLYTE PREPARED BY TREATING CHROMITE WITH SULPHURIC ACID AND CHROMIC ACID, THE STEPS OF UTILIZING SULPHURIC ACID AND CHROMIC ACID DERIVED FROM THE ANOLYTE OF THE COMPARTMENT CELL TO PROVIDE AT LEAST PART OF THE NECESSARY SULPHURIC AND CHROMIC ACIDS, DIGESTING THE ACIDS AND CHROMITE AT APPROXIMATELY 120*C., SEPARATING THE CHROMIC SULFATE AND OTHER METALLIC SULFATES DISSOLVED FROM THE CHROMITE FROM THE INSOLUBLE RESIDUE, ADDING OM AMMONIUM SULFATE IN AN AMOUNT SUFFICIENT TO FORM AMMONIUM SULFATE COMPLEXES OF THE CHROMIUM AND OTHER METALLIC SULFATES PRESENT, HEATING THE SOLUTION TO A TEMPERATURE ABOVE 70*C. AND BELOW THE TEMPERATURE AT WHICH DECOMPOSITION OCCURS, WHEREBY TO STABLIZE THE CHROMIUM SALTS IN THE GREEN MODIFICATION, COOLING THE SOLUTION TO A TEMPERATURE BETWEEN **-5* AND **-10*C. TO CRYSTALLIZE IRON,. ALUMINUM AND MAGNESIUMAMMONIUM SULFATE COMPLEXES AND SEPARATING THEM FROM THE SOLUTION OF GREEN CHROMIUM SULFATE, SEEDING THE SOLUTION TO OBTAIN THE CHROMIUM SALTS IN THE VIOLET MODIFICATION, ADDING A SUBSTANTIAL FRACTION OF THE SPENT CATHOLYTE TO THE SOLUTION TO CATALYZE THE CRYSTALLIZATION OF VIOLET CHROMIUM ALUM FROM THE SOLUTION, RE-DISSOLVING THE CRYSTALLIZED CHROMIUM ALUM TO FORM AN AQUEOUS SOLUTION, MIXING THE CHROMIUM ALUM WITH SODIUM SULFATE IN AN AMOUNT SLIGHTLY LESS THAN THE AMMONIA CONTENT OF THE ALUM TO FORM A CATHOLYTE SOLUTION HAVING 20-40 GRAMS PER LITER OF SODIUM, 30-50 GRAMS PER LITER OF AMMONIUM, AND 18-60 GRAMS PER LITER OF CHROMIUM, ALL IN THE FORM OF SULFATES, HEATING THE CATHOLYTE SOLUTION TO ABOVE 70*C. AND BELOW THE TEMPERATURE AT WHICH DECOMPOSITION OCCURS TO STABLIZE THE GREEN MODIFICATION, ELECTROLYZING SUCH SOLUTION IN THE CATHOLYTE COMPARTMENT OF AN ELECTROLYTIC CELL AND FORMING IN THE ANOLYTE COMPARTMENT OF SUCH CELL CHROMIC AND SULPHURIC ACIDS. 